1. Field of the Invention
This invention relates to a watercolor ink most suitable for ink jet printing devices, writing implements, recording meters, and stamping devices. More particularly, this invention relates to a watercolor ink for use in ink jet printing devices, which watercolor ink avoids clogging the ink jet nozzle, jets stably from the nozzle, dries quickly after application to a surface, precludes feathering and strike through, and produces prints of high quality.
2. Prior Art Statement
Ink jet printing is quiet, enables increased printing speed, makes color printing possible, and allows use of plain paper for the printing. These features have encouraged development of this printing system for application to various types of printers and facsimile machines.
In the ink jet printing system, there have been proposed a number of specific printing methods using different combinations ink drop control methods and ink drop jetting methods. Further, both oil color inks and watercolor inks are used in these printing methods. The watercolor ink now in use has a water-soluble dye, a water-soluble organic solvent, and water as essential ingredients.
In the ink jet printing system mentioned above, however, the ink must be jetted continuously and stably in the form of uniform drops through very minute nozzle orifices. The stability of ink drop jetting and the freedom of the nozzle from clogging with the ink are major requirements which the watercolor ink is expected to fulfill.
Further, properties of the watercolor ink which become important after application of the ink to the printing paper are, for example, its drying speed, its strike through and its feathering.
Further, in the ink jet printing system, when plain paper is used, the conventional watercolor ink permeates the paper very slowly and the printing formed by the ink on the surface of the paper does not dry quickly because the ink has high surface tension and the paper has a sizing agent on its surface. Thus, this system prevents use of plain water as the printing paper and requires use of paper having a special surface coating.
To permit use of plain paper for the printing, the method which comprises increasing the alkalinity of the ink and having a sizing agent dissolved in the ink thereby heightening the apparent drying speed of the ink has been proposed. In this case, the problems of strike through and feathering are not entailed. Since the ink has a pH value of not less than 13, however, there is a possible health hazard to humans and color printing is impossible.
It is conceivable to heighten the apparent drying speed of the ink applied to plain paper by incorporating a surfactant in the ink thereby heightening the permeability of the ink through the paper. In this case, the ink entails the problems of strike through and feathering because it has low surface tension and permeates the paper in such a manner as to fill up the interstices in the paper.
For a solution of the problem concerning the clogging of the nozzle orifices with the ink, Japanese Patent Public Disclosure SHO 62-164773 discloses an ink which incorporates therein, in the form of an emulsion, an oily substance insoluble in the essential components of ink. Though this ink solves the problem of clogging, it still suffers from poor solubility and inferior stability because it is an emulsion in state.
For the prevention of the nozzle orifices from the trouble of clogging, Japanese Patent Publication SHO 62-15594 discloses an ink composition which uses diethylene glycol monobenzyl ether in addition to the essential ingredients of ink, i.e. a dye and water. This ink composition, however, is still deficient in stability of continuous jetting at low temperatures (near 5.degree. C.).
In Japanese Patent Publication SHO 60-23793, there is disclosed an ink composition which uses an aliphatic polyhydric alcohol as a wetting agent for the purpose of preventing the nozzle orifices from clogging. This ink composition has a disadvantage in that it entails feathering and strike through and the print formed of the ink composition lacks clarity.
As described above, when the watercolor ink is positively furnished with enough wettability to solve the problem of clogging of the nozzle orifices with the ink, it is compelled to sacrifice quick-drying property and suffers from feathering and strike through. It is very difficult to produce an ink which fulfills all of the requirements.